The code for current-kill

The current-kill function is used by yank and by
yank-pop. Here is the code for current-kill:

(defun current-kill (n &optional do-not-move)
"Rotate the yanking point by N places, and then return that kill.
If N is zero, `interprogram-paste-function' is set, and calling it
returns a string, then that string is added to the front of the
kill ring and returned as the latest kill.
If optional arg DO-NOT-MOVE is non-nil, then don't actually move the
yanking point; just return the Nth kill forward."
(let ((interprogram-paste (and (= n 0)
interprogram-paste-function
(funcall interprogram-paste-function))))
(if interprogram-paste
(progn
;; Disable the interprogram cut function when we add the new
;; text to the kill ring, so Emacs doesn't try to own the
;; selection, with identical text.
(let ((interprogram-cut-function nil))
(kill-new interprogram-paste))
interprogram-paste)
(or kill-ring (error "Kill ring is empty"))
(let ((ARGth-kill-element
(nthcdr (mod (- n (length kill-ring-yank-pointer))
(length kill-ring))
kill-ring)))
(or do-not-move
(setq kill-ring-yank-pointer ARGth-kill-element))
(car ARGth-kill-element)))))

Remember also that the kill-new function sets
kill-ring-yank-pointer to the latest element of the kill
ring, which means that all the functions that call it set the value
indirectly: kill-append, copy-region-as-kill,
kill-ring-save, kill-line, and kill-region.